Inter Partes Review (IPR) proceedings are limited to prior art challenges for printed prior art. Although prior art typically includes drawings that illustrate physical structures, the description of those drawings may only discuss what is illustrated and omit any details surrounding the distribution of mass or other physical qualities of the objects depicted in the drawings. The PTAB recently discussed the level of detail required to support a prior art challenge to patentability when the prior art omits a substantial physical description of what is depicted in the prior art drawings.